Hydrogels with three-dimensional cross-linked networks and high capacity of water or biological fluid have been intensively investigated for their potential biomedical applications in controlled drug delivery devices, micro fluidic devices, biosensors, tissue implants, and contact lenses. Most synthetic hydrogels suffer from poor mechanical properties (strength, extensibility, toughness, and recovery), due to their intrinsic structural heterogeneity and/or lack of efficient energy-dissipation mechanisms. Development of strong and flexible hydrogels with novel microstructures and excellent mechanical properties is still a challenging task. Many efforts have been made to design novel hydrogels with improved mechanical properties, such as double-network hydrogels, slide-ring hydrogels, nanocomposite hydrogels, triblock copolymers hydrogels, hydrophobic modified hydrogels, tetra-PEG gels, and macromolecular microsphere composite hydrogels.
Among them, double-network hydrogels consisting of two cross-linked networks with strong asymmetric structures have demonstrated to achieve improved and balanced mechanical properties between strength and toughness by tuning inter/intramolecular interactions and structures within and between two networks using a wide variety of polymeric monomers, cross-linkers, and cross-linked methods. Double-network hydrogels are usually synthesized via a multi-step sequential free-radical polymerization process. First, strong polyelectrolytes such as poly(2-aclylamido, 2-methyl,1-propanesulfonic acid) (PAMPS) are often used to form a highly covalently cross-linked, rigid and brittle, first network. Due to the highly swelling nature of strong polyelectrolytes, upon immersion of the polyelectrolyte hydrogels into a precursor solution containing neutral second monomers, initiators, and cross-linkers for the second polymerization, these reactants will diffuse into and react with the first brittle network to form a loosely cross-linked, soft and ductile, neutral, second network. These multi-step polymerization methods have demonstrated the feasibility to produce different high strength hydrogels, such as microgel-reinforced hydrogel, void double-network gels, inverse double-network gels, jellyfish gels, liquid crystalline double-network gels, and lamellar bilayers double-network gels.
However, use of multi-step methods for preparing double-network gels still encounters some challenges and limitations. Some challenges are (1) process tedious and time-consuming, which involves swelling, diffusion, and two polymerization processes, and also requires 1-2 days to complete the DN gels. (2) difficult to control, optimize, and determine experimental conditions. Due to the uncontrollable swelling and diffusion processes, it is rather difficult to control the exact mole ratio of the two networks and to reproduce the gels with similar mechanical properties even using the same conditions. This also causes a large amount of waste for unreacted second-network monomers. (3) lack of shape-flexibility. The two-step methods can not straightforward prepare different complex-shaped gels due to involvement of a swelling process. (4) Lack of self-recovery properties of DN gels. Most of DN hydrogels are chemically linked for both networks, which make the gels very difficult to be repaired and recovered from damages and fatigues, simply because of irreversible bond breaking. Several attempts have been made to overcome some of these drawbacks such as by modifying the first network/second network structure to increase their self-recovery properties or trying to simplify the preparation methods for the precursor reactants to diffuse into and react with the first network. However, all of the attempts still involved two or three polymerization step and an additional one to two swelling steps.
Therefore there is a need in the art for a novel, simple, and robust one-pot method to overcome these drawbacks which allow for the design and synthesis of a new type of hydrogels with highly mechanical and recoverable properties.